Many attempts have been made to derive useful energy from the wind. In general, design has progressed to the point that, on a per-energy-unit cost basis, wind turbine energy costs approach those of conventional power sources. Because of the size of worldwide power needs, even small incremental changes in wind turbine efficiency can provide substantial benefits.
One of the factors influencing the efficiency of a wind turbine is the lift-to-drag ratio of the blades. The lift-to-drag ratio can be improved by increasing the lift or decreasing the drag. Drag can be reduced by reducing the size of the blades, i.e. making "thinner" blades. However, the blades must be strong enough to withstand the mechanical load on the blades. Accordingly, it would be useful to provide a wind turbine which had decreased load on the blades since this would permit use of thinner blades, thus increasing lift-to-drag ratio.
Another factor affecting the operation and economics of wind turbines is variability. The variable nature of wind has, in the past, required devices having sufficient structural strength to withstand the peak loads such as surges in the amount of torque developed (e.g., from wind gusts and the like). Thus, any previous devices required large and strong structures to accommodate, e.g., torque surges, even though, during the majority of the time such large and strong structures are not needed (i.e. during non-gust events). Furthermore, torque surges result in undesirable power surges. Accordingly, it would be useful to provide a wind turbine which can effectively control torque surges and thus reduce the occurrence of power spikes and reduce the need for heavy and strong structures.
Some previous wind turbine designs provided for pivoting or "teetering" of the blade structure (i.e., pivoting of the blades, as a unit) with respect to the axis of rotation, e.g., to accommodate non-uniform wind inflow conditions such as wind sheer. However, some approaches for limiting the amount of teeter motion tend to provide an undesirable amount of stress on parts, sometimes leading to fatigue and/or failure of components. Further, a teetering wind turbine can have instability in low velocity wind conditions or during a high wind velocity restart. Accordingly, it would be useful to provide a wind turbine in which, if blade teetering is used, teetering can be effectively controlled while reducing or eliminating fatigue or failure of components.
An important aspect of many wind turbine devices is the design of the tower on which the rotor and generator device are mounted. Certain previous tower designs, particularly those intended for transportable or mobile installations, permitted pivoting of all or a portion of the tower about a pin or other axis. While useful for some purposes, this design has resulted in a tower which has different stiffness in different directions. Accordingly, it would be useful to provide a tiltable tower structure which can reduce or eliminate the directionally differential stiffness of the tower structure.